Streams and files - Michigan State Universitycse232/Weekly/week05/readings/8... · 2021. 1. 11. ·...

10/1/17

1

COMPUTER SCIENCE DEPARTMENT

Streams and files


more on streams

10/1/17

2


streams, files, stringstreams

IO Streams

Monitor

Executingprogram Keyboard

Mouse

ostream istream

Output Device Input Device

Streams are objects with names such as cin, cout, cerr.


Each stream has an associated buffer, part of the stream object, that data is pulled/pushed from

Buffer


10/1/17

3



istream

Executingprogram 18

cin

The >> (extraction) operator is an overloaded operator that takes values out of an input stream (cin), and stores them as the type indicated by the target variable.int my_var; cin >> my_var;The '1' is read, and then the '8' is read.Then the two characters are converted to the integer 18which is stored in variable my_var.


When cin goes bad

Executingprogram abc123…

cin buffer

int my_int=10; cin >> my_int; //

a b c 1 2 3 …

failtyped operator, can only readint type stuff, fails at the 'a'


10/1/17

4


fail is fail, you must fix

Executingprogram abc123 …

cin buffer

int my_int=10;char my_char='a';cin >> my_int; // failcin >> my_char;// fail

a b c 1 2 3 …

fail

cin stays in failed state until you clean it up. All subsequentreads fail until that happens



• Useful boolean member functions:cin.good(): all is well in the istream

cin.bad(): something is wrong with istream

cin.fail(): last op could not be completed

cin.eof(): last op encountered end-of-file

• Useful with the assert() function: e.g. assert(cin.good())


Status Functions

10/1/17

5


cin.clear()

Executingprogram abc123 …

cin buffer

int my_intcin >> my_int; if(cin.fail())

cin.clear();cin >> an_int;

a b c 1 2 3 …

goodclear clears the error,back to good, but notthe problem. Buffer isunchanged!!! Fails again.

fail



cin.ignore(num_chars_to_skip, stop_char)

where num_chars_skip clears that number of chars from the buffer up to and including stop_char

E.g. cin.ignore(20,'\n') skip 20 characters or until '\n' whichever comes first


clear the buffer

10/1/17

6


clear, then ignoreExecutingprogram empty

cin buffer

int my_int, an_int;cin >> my_int;if(cin.fail()){

cin.clear();cin.ignore(1000, '\n');

}// reprompt, try again

goodignore empties thebuffer. Now you cantry again (repromptfor example).

empty buffer



• takes a default count as 1• any number works• numeric_limits::max()

(requires #include) means as many as necessary to hit the stop char.

• takes a default stop as the eof char

more on ignore


10/1/17

7


The situation is more complicated for numbers. For example, try reading a float into an integer.

more complicated for a float



When cin goes bad

Executingprogram 18.123…

cin buffer

int my_int; cin >> my_int; //18

1 8 . 1 2 3 …

not a failure, more likea separator

typed operator, can only readint type stuff, stops (not fails) at the '.'


10/1/17

8


When cin goes bad

Executingprogram 18.123…

cin buffer

int my_int, an_int; cin >> my_int; //18 cin >> an_int;

1 8 . 1 2 3 …

fail

next read is a failure (chokes on the '.')



When cin goes bad

Executingprogram 18\n hi\n

cin buffer

int my_int, an_int; cin >> my_int; //18 cin >> an_int;

1 8 \n h i \n …

fail

next read is a failure (chokes on the '.')


10/1/17

9


We'll see it is easier to treat this as a string and try to cast it.

Better to treat as a string and cast



cin >> some_var returns:

• cin if things go well• false if you hit eof• false if the stream is in a fail or bad mode

Thus you can:while(cin >> some_var)

…

cin returns ?


10/1/17

10


• White space: blanks, tabs, and returns• By default, the >> operator skips

leading white space• int X, Y, Z; • cin >> X >> Y >> Z;• Input: 3 4 5X is 3, Y is 4, Z is 5


White space


• Turn off skipping white space: • cin >>noskipws

• Turn skipping white space back on: • cin >> skipws

• ALTERNATIVE: use an input function which does not skip white space: cin.get(ch) reads exactly one character no matter the character


Controlling White Space

10/1/17

11


• To read a single character, not skipping:• cin.get(ch)

• To put that character back into the buffer• cin.putback(ch)

• To peek without removing it:• cin.peek()


Single Character


• Single character function: cout.put(ch) puts a single character into the ostream.


Output functions

10/1/17

12



ostream

Monitor

Executingprogram

ostream

Output Device



cout and cerr

Monitor

Executingprogram

Output Device

cout

cerr

Cout and cerr are particular instances of an ostream.

10/1/17

13



10/1/17

14


• Output characters are stored into a buffer before being output, i.e. gather up a bunch of characters before sending them to the screen. This can be a problem for debugging: output may be in the buffer leading you to believe that an error occurred before the output statement when it actually occurred afterward.


Buffer


double f(double X){cout

10/1/17

15


double f(double X){cout

10/1/17

16


• Files are collections of data and are stored in nonvolatile memory, e.g. secondary storage such as disk.

• Text Files store characters such as ASCII, e.g. source code.

• Binary Files contain non-ASCII characters, e.g. compiled code.

• Humans can read text files.


Files



Stream Review

Monitor

Executingprogram Keyboard

Mouse

ostream istream

Output Device Input Device

Streams are objects with names such as cin, cout, cerr.

10/1/17

17



File Streams

Executingprogram

ofstream ifstream

Output Input

Previous streams were objects with names such as cin, cout, cerr.Now we add streams which are files. We can name them.

File (on disk)File (on disk)


Because we are working with the stream object, the pipe, we do not have to worry about particular devices (that is the software's problem).

Result is that many of the operations we used with cin and cout work with files.

just another stream


10/1/17

18


• required #include. This provides two kinds:• ifstream (input files)• ofstream (output files)

• Can establish a connection by:• declare with the name (as a string) to open

automatically• .open(string) method to establish

connection between a program and a file.


to work with a file


#include// automatically open in_file

ifstream in_file("my_file.txt");

ofstream out_file;

string file_name;

cin >> file_name;

// out_file created and now openedout_file.open(file_name);

example


10/1/17

19


When you open a file with a simple name, like "file.txt", the assumption is that the file is located in the same director/folder as the executing program.

If not there, you have to give a fully qualified path.

Where is that file?



Sadly, this can depend on the underlying operating system:

• C:\Documents\My Folder\file.txt• /usr/local/bill/file.txt

Know that it is true and assume the file is in the correct place

fully qualified path


10/1/17

20


• >>,

10/1/17

21


other file modes

for input files, thedefault is input

for output files, thedefault is open andtrunc. Thus outfiles, by default, wipeout any info in thefile being written to



If you declare a file as an fstream, you get to decide what aspects you want.

fstream in_out_file ("file.txt", fstream::in | fstream::out | fstream::ate);

This a file one can read from and write to, and writing occurs at the end of the file.

specify yourself

vertical bars are bitwise or operator,look it up. We combine all aspects this way


Streams and files - Michigan State Universitycse232/Weekly/week05/readings/8... · 2021. 1. 11. ·...

Documents

Transcript of Streams and files - Michigan State Universitycse232/Weekly/week05/readings/8... · 2021. 1. 11. ·...